1. Field of the Invention
The invention is generally directed to semiconductor fabrication. The invention is more specifically directed to the formation of a buried isolation region using implantation of oxygen, nitrogen or other insulation-forming particles into a single crystal semiconductor substrate.
2. Description of the Related Art
It is sometimes desirable, in the fabrication of integrated circuit (IC) devices, to define an insulator region buried deep within a semiconductor substrate. The insulator region provides electrical isolation between a top portion of the substrate (epi region) and a deeper portion of the substrate (bulk region).
The term "electrical isolation" is used here to mean that one or more of the following conditions are met: (a) a relatively high resistance separates the epi region electrically from the bulk region; (b) a relatively small capacitance separates the epi region electrically from the bulk region; and (c) the electrical separation of the epi region from the bulk region is able to withstand high voltages without breakdown. Resistance is maximized, capacitance is minimized and breakdown voltage is maximized by providing a relatively thick insulator region of homogenous high resistivity and homogenous low dielectric constant between parts of the substrate that are to be isolated from one another.
Traditional isolation techniques create a homogenous high doping concentration of insulative molecules within the substrate. In a silicon (Si) substrate for example, a concentration of approximately 2.times.10.sup.18 atoms/cm.sup.2 of oxygen is provided uniformly across a buried insulator region of one-half micron (0.5 .mu.m) or greater thickness. High energy implantation is used to introduce oxygen atoms into a subsurface region of a semiconductor substrate. When the substrate is monocrystalline silicon, the silicon atoms combine with the implanted O atoms to produce SiO.sub.2.
Implantation of the oxygen atoms initially produces a very thin region (e.g., 0.1 .mu.m or less) of extremely high concentration. Post-implant heat treatment diffuses the implanted particles over a broader region (0.5 .mu.m). The resultant concentration of insulation-forming particles then assumes a Gaussian-like (bell-shaped) distribution relative to the initial implant depth.
Practitioners conventionally implant equal concentrations of oxygen atoms at a series of equally-spaced levels below the substrate surface in order to create a thick homogenous insulator region. During post-implant heat treatment, the Gaussian diffusion distributions of the multiple implants overlap to produce a nearly homogenous concentration of insulation-forming atoms.
There is a drawback to this method, however. Each high-energy implantation disadvantageously creates defects in the crystal structure of the region through which it passes. The number of defects tends to increase as the concentration of implant atoms increases. The number of defects also tends to increase as the implant energies increase.
When buried insulator regions of homogenous composition are produced within a semiconductor device by way of high energy implantation, the deepest implant causes the most damage to the crystal structure of the overlying epi region. Progressively less but nonetheless cumulative damage is produced by the high energy implantation of atoms into progressively shallower depths.
The resultant damage to the crystal structure of the epi layer can interfere with the operability of electronic circuits that are defined in the epi region either at later or earlier times. Production yields can become unacceptably low. The cost of fabricating circuits with implant-defined insulator regions then becomes unattractively high.
A need exists in the industry for a high yield fabrication method that can provide electrical isolation by way of high energy implantation.